DIAGNOSIS You are using the legacy 'Nighthawk' app. The RAX50 requires the newer 'Nighthawk' or 'Orbi' app. The old app is incompatible with newer WiFi 6 firmware.
FIX Uninstall the old Netgear app from your phone. Go to your app store and search for 'Netgear Nighthawk'. Install the official app published by 'Netgear, Inc.'. Ensure your phone is connected to the router's WiFi. Open the new app and follow the setup prompts. It will discover the router via local network broadcast.
IF THAT DIDN'T WORK Local discovery is blocked. On your phone, disable mobile data temporarily. Ensure you are connected to the RAX50's 2.4GHz network for initial setup (5GHz can sometimes fail). Also, check that 'AP Isolation' or 'Guest Network' is not enabled on the band you're connected to, as this blocks device communication.
HEADS UP The app requires local network permissions. On iOS, enable 'Local Network' for the app in Settings > Privacy & Security > Local Network. On Android, allow all permissions the app requests during setup. The app will not work remotely unless you set up a Netgear account and enable remote management in the router's web interface.